Conformal coating composition containing metal nanoparticles to prevent sulfur related corrosion

ABSTRACT

A conformal coating composition for protecting a metal surface from sulfur related corrosion includes a polymer and metal nanoparticles blended with the polymer. In accordance with some embodiments of the present invention, an apparatus includes an electronic component mounted on a substrate, metal conductors electronically connecting the electronic component, and a polymer conformal coating containing metal nanoparticles overlying the metal conductors. Accordingly, the metal nanoparticle-containing conformal coating is able to protect the metal conductors from corrosion caused by sulfur components (e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in the air. That is, the metal nanoparticles in the conformal coating react with any corrosion inducing sulfur component in the air and prevent the sulfur component from reacting with the underlying metal conductors.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation application of pending U.S.patent application Ser. No. 14/156,635, filed Jan. 16, 2014, entitled“CONFORMAL COATING COMPOSITION CONTAINING METAL NANOPARTICLES TO PREVENTSULFUR RELATED CORROSION”, now U.S. Pat. No.: US 10,479,897 B2, which ishereby incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates in general to the field of corrosionprotection. More particularly, the present invention relates to apolymer conformal coating composition containing metal nanoparticlesproviding corrosion protection for metal surfaces against corrosioncaused by environmental sulfur components.

SUMMARY

In accordance with some embodiments of the present invention, aconformal coating composition for protecting a metal surface from sulfurrelated corrosion includes a polymer and metal nanoparticles blendedwith the polymer. In accordance with some embodiments of the presentinvention, an apparatus includes an electronic component mounted on asubstrate, metal conductors electronically connecting the electroniccomponent, and a polymer conformal coating containing metalnanoparticles overlying the metal conductors. Accordingly, the metalnanoparticle-containing conformal coating is able to protect the metalconductors from corrosion caused by sulfur components (e.g., elementalsulfur, hydrogen sulfide, and/or sulfur oxides) in the air. That is, themetal nanoparticles in the conformal coating react with any corrosioninducing sulfur component in the air and prevent the sulfur componentfrom reacting with the underlying metal conductors.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present invention will hereinafter be described inconjunction with the appended drawings, where like designations denotelike elements.

FIG. 1 is an exploded view of a gate resistor of a resistor networkarray that utilizes a conformal coating composition containing metalnanoparticles to protect metal conductors in accordance with someembodiments of the present invention.

FIG. 2 is a sectional view of the gate resistor shown in FIG. 1, butwhich is shown mounted on a printed circuit board.

FIG. 3 is a top view of a resistor network array mounted on a printedcircuit board that utilizes a conformal coating composition containingmetal nanoparticles to protect metal conductors in accordance with someembodiments of the present invention.

FIG. 4 is a flow chart diagram of a method for producing a resistornetwork array that utilizes a conformal coating composition containingmetal nanoparticles to protect metal conductors in accordance with someembodiments of the present invention.

DETAILED DESCRIPTION

The electronics industry designs and tests hardware to be able towithstand typical indoor environments. Hardware failures can occur,however, in geographies with harsher indoor environments than the designset point. This has resulted in electronic component failure due tocorrosion of metallurgy via a corrosive gas environment. Attempts tomitigate these electronic component failures have focused on the use ofcommercially available conformal coatings. These conformal coatings fallinto several generic classes: silicones, epoxies, acrylates, and otherorganic materials. However, accelerated aging testing has revealed thatsilicones may actually exacerbate the problem and that corrosion ismerely retarded by the other classes of conformal coatings. Furthermore,studies have revealed sulfur components (e.g., elemental sulfur, H₂S,and sulfur oxides) in the gaseous environment as the major culprit. Ofthe sulfur components, elemental sulfur appears to be the mostaggressive.

In accordance with some embodiments of the present invention, aconformal coating composition for protecting a metal surface from sulfurrelated corrosion includes a polymer (e.g., a commercially availableconformal coating, such as Dow Corning® 1-2620 RTV Coating) and metalnanoparticles (e.g., copper nanoparticles) blended with the polymer.Copper nanoparticles, for example, added to the polymer as a fillermaterial would react with elemental sulfur, for example, in the gaseousenvironment to form CuS and/or Cu₂S at room temperature. This wouldprevent the corrosive agents from reacting with the underlyingelectronic component metallurgy, thus extending the product life.

The metal nanoparticles may be blended into the polymer using a highshear dispersion mixer or other suitable technique known to thoseskilled in the art.

Conformal coatings typically fall into several generic classes:silicones, epoxies, acrylates, and other organic materials. Hence, aconformal coating composition in accordance with some embodiments of thepresent invention includes a polymer that may be, for example, one ormore silicone-based polymers, one or more epoxy-based polymers, one ormore acrylate-based polymers, and/or one or more other organicmaterials; and combinations thereof. For example, the polymer may be aconventional RTV silicone rubber composition, such as Dow Corning®1-2620 RTV Coating or Dow Corning® 1-2620 Low VOC RTV Coating. Moregenerally, the polymer may include, but is not limited to, any suitablepolysiloxane, polyepoxide, polyacrylate and/or other organic polymericmaterial; and combinations thereof. Typically, it is desirable for thepolymer to be non-water absorbing to avoid shorting from occurringthrough pathways created by water. It may also be desirable for thepolymer to be halogen-free (i.e., RoHS compliant).

Metal nanoparticles suitable for blending into the polymer in accordancewith some embodiments of the present invention will selectively bind toelemental sulfur, thus preventing corrosion of the underlying metalstructures. Suitable metal nanoparticles are sulfur-getters that preventthe target corrosive species (e.g., elemental sulfur in the air) fromever reaching the metallurgy by which the electronic component iselectrically connected, thus eliminating possible corrosion. Suitablemetal nanoparticles include, but are not limited to, coppernanoparticles, silver nanoparticles and nanoparticles of any othersuitable metal that reacts with elemental sulfur; and combinationsthereof. Copper nanoparticles, for example, will react with elementalsulfur to form CuS and/or Cu₂S at room temperature. Suitable metalnanoparticles are typically high purity metal (e.g., 99.8% purity coppernanoparticles).

Suitable metal nanoparticles may have any size and shape. Typically,suitable metal nanoparticles will have an average size (diameter) withinthe range of approximately 5 nm to 200 nm. Smaller metal nanoparticlesare usually more reactive than larger ones of the same type. Particlesize also impacts the electric conducting percolation threshold,discussed below. Suitable metal nanoparticles may be spheres, plates,wires, rods, tubes, or any other shape.

Suitable metal nanoparticles are commercially available from vendorssuch as Sigma-Aldrich Co. LLC and SkySpring Nanomaterials, Inc.

The metal nanoparticles can be blended into the conformal coatingcomposition as a filler material at concentrations below the electricconducting percolation threshold (typically, less than approximately 10wt %). This percolation threshold must not be exceeded to avoid shortingof the PCB to be coated.

The electric conducting percolation threshold is related to the metalnanoparticle volume fraction in the conformal coating composition. It isknown that electric conducting percolation takes place when the particlevolume fraction of particles in a polymer/conducting particle compositereaches the critical volume fraction ψ_(C). The critical volume fractionis given by the following equation:

$\psi_{C} = {\frac{1}{6}\pi\; D^{3}\frac{N_{c}}{V}}$wherein D is the average diameter of the conducting particles, V is thevolume of the polymer/conducting particle composite, and N_(C) is thenumber of conducting particles when the electric percolation takesplace. See, X. Jing et al., “The effect of particle size on electricconducting percolation threshold in polymer/conducting particlecomposite”, Journal of Materials Science Letters, Volume 19, Issue 5,2000, pp. 377-379.

Typically, the concentration of the metal nanoparticles in the conformalcoating composition is selected to provide a desired longevity (e.g.,the life expectancy of a computer system in which the PCB to be coatedis installed) of sulfur-gettering functionality and yet maintain levelsof electrical conduction comfortably below the percolation threshold.For example, a 5 wt % loading concentration of 100 nm Cu nanoparticlesin the conformal coating composition will be able to continue to reactover the 7+ year life of a typical computer system.

EXAMPLE Prophetic

0.05 g of 100 nm copper nanoparticles are blended with 1 g of DowCorning® 1-2620 RTV Coating using a high shear dispersion mixer or othersuitable technique known to those skilled in the art. Coppernanoparticles (100 nm diameter) have been shown to react quantitativelywith a 1:1 atomic mass ratio of sulfur. Therefore, a conformal coatingcontaining 0.05 g of 100 nm Cu nanoparticles can react with a maximumsulfur concentration of 0.025 g, which is substantially higher than theppb concentration of sulfur typically present in the environment of adata center. With the low, but damaging, concentration of sulfur in adata center, the copper nanoparticles will be able to continue to reactover the 7+ year life of a typical computer system.

The above-listed exemplary metal nanoparticles, polymers andconcentrations are set forth for the purpose of illustration, notlimitation. Those skilled in the art will appreciate that other metalnanoparticles, polymers, and/or concentrations may be used within thescope of the present invention.

In accordance with some embodiments of the present invention, anapparatus includes an electronic component mounted on a substrate, metalconductors electronically connecting the electronic component, and aconformal coating containing a polymer (e.g., a commercially availableconformal coating, such as Dow Corning® 1-2620 RTV Coating) and metalnanoparticles (e.g., copper nanoparticles) overlying the metalconductors. Accordingly, the metal nanoparticle-containing conformalcoating is able to protect the metal conductors from corrosion caused bysulfur components (e.g., elemental sulfur, hydrogen sulfide, and/orsulfur oxides) in the air. That is, the metal nanoparticles in theconformal coating react with any corrosion inducing sulfur component inthe air and prevent the sulfur component from reacting with theunderlying metal conductors.

Corrosion caused by sulfur components (e.g., elemental sulfur, hydrogensulfide, and/or sulfur oxides) in the air is especially severe when oneor more of the metal conductors that electrically connect an electroniccomponent is/are a silver-containing metal. For example, each of thegate resistors of a resistor network array typically utilizes a silverlayer at each of the gate resistor's terminations. Gate resistors arealso referred to as “chip resistors” or “silver chip resistors”.Typically, gate resistors are coated with a glass overcoat for corrosionprotection. Also for corrosion protection, it is known to encapsulategate resistors in a resistor network array by applying a coating of aconventional room temperature-vulcanizable (RTV) silicone rubbercomposition over the entire printed circuit board on which the resistornetwork array is mounted. However, the glass overcoat and conventionalRTV silicone rubber compositions fail to prevent or retard sulfurcomponents in the air from reaching the silver layer in gate resistors.Hence, any sulfur components in the air will react with the silver layerin the gate resistor to form silver sulfide. This silver sulfideformation often causes the gate resistor to fail, i.e., the formation ofsilver sulfide, which is electrically non-conductive, produces anelectrical open at one or more of the gate resistor's terminations.

The use of silver as an electrical conductor for electrically connectingelectronic components is increasing because silver has the highestelectrical conductivity of all metals, even higher than copper. Inaddition, the concentration of sulfur components in the air isunfortunately increasing as well. Hence, the problem of corrosion causedby sulfur components in the air is expected to grow with the increaseduse of silver as an electrical conductor for electrically connectingelectronic components and the increased concentration of sulfurcomponents in the air.

Some embodiments of the present invention are described herein in thecontext of protecting metal conductors of an exemplary gate resistor ina resistor network array from corrosion caused by sulfur components inthe air. One skilled in the art will appreciate, however, that thepresent invention can also apply to protecting metal conductors of gateresistors and resistor network arrays having configurations differingfrom the gate resistor and resistor network array shown in FIGS. 1-3 andto protecting metal conductors of other electronic components, and, moregenerally, to protecting a metal surface of any product. For example,the present invention can be used to protect controlled collapse chipconnection (C4) solder joints that electrically connect terminals orpads on the base of a flip-chip with corresponding terminals or pads ona module substrate.

Referring now to FIG. 1, there is depicted, in an exploded view, a gateresistor 100 of a resistor network array 300 (shown in FIG. 3) thatutilizes a polymer conformal coating 130 containing metal nanoparticles,which according to the some embodiments of the present invention,provides corrosion protection for metal conductors. FIG. 2 is asectional view of the gate resistor 100 shown in FIG. 1, but which isshown mounted on a printed circuit board 210. FIG. 3 is a top view of aresistor network array 300 that utilizes the metalnanoparticle-containing conformal coating 130 shown in FIGS. 1 and 2.

As shown in FIGS. 1 and 2, a resistor element 102 is mounted to asubstrate 104, such as a ceramic substrate. The gate resistor 100includes two termination structures 110, each typically comprising aninner Ag (silver) layer 112, a protective Ni (nickel) barrier layer 114,and an outer solder termination layer 116. Each of the terminationstructures 110 of the gate resistor 100 is also referred to herein as a“metal conductor”.

Typically, for corrosion protection, each gate resistor in a resistornetwork array is coated with a conventional protective coating, such asa glass overcoat 120.

The gate resistors in a resistor network array are typically soldered toa printed circuit board by SMT (surface mounting technology) processes.As best seen in FIG. 2, the termination structures 110 of each gateresistor 100 in the resistor network array 300 (shown in FIG. 3) aresoldered to corresponding terminals or pads 212 on the printed circuitboard 210. For example, the outer solder termination layer 116 of thetermination structures 110 of each gate resistor 100 may be reflowed tojoin (i.e., electrically and mechanically) the termination structures110 on the base of the gate resistor 100 with the correspondingterminals or pads 212 on the printed circuit board 210.

As best seen in FIG. 3, in accordance with some embodiments of thepresent invention, the metal nanoparticle-containing conformal coating130 covers essentially the entire printed circuit board 210,encapsulating each of the gate resistors 100 of the resistor networkarray 300 (as well as any other discrete electronic component(s) mountedon the board 210). Hence, the metal nanoparticle-containing conformalcoating 130 overlies the metal conductors 110 of the gate resistor 100to provide corrosion protection, i.e., the metal nanoparticle-containingconformal coating 130 protects the metal conductors 110 of the gateresistor 100 from corrosion caused by sulfur components (e.g., elementalsulfur, hydrogen sulfide, and/or sulfur oxides) in the air.

Alternatively, the metal nanoparticle-containing conformal coating 130may cover only one or more specific areas of the printed circuit board210 that is/are susceptible to corrosion caused by sulfur components inthe air (e.g., the area of the printed circuit board 210 encompassingthe resistor network array 300).

The metal nanoparticle-containing conformal coating 130 has sulfurgettering functionality which can significantly extend the product lifewhen the gate resistor 100 (or other electronic component) is to be usedin a corrosive gas environment. This benefit of some embodiments of thepresent invention is achieved without affecting the operation of thegate resistor 100 (or other electronic component).

Advantageously, existing deposition processes may be used for applyingthe metal nanoparticle-containing conformal coating 130 to the printedcircuit board 210, and thereby encapsulate the resistor network array300 and other discrete electronic component(s) mounted on the printedcircuit board 210. The present invention may be implemented in anycurrently used conformal coating process utilized in the preparation ofelectronic components. Numerous processes conformally coat componentswith polymers. Metal nanoparticles may be blended with these polymerswithin the scope of the present invention. In effect, a metalnanoparticle-containing polymer conformal coating in accordance withsome embodiments of the present invention replaces a conventionalpolymer conformal coating. Typically, there would be neither asignificant change in the processing of components nor a significantchange in the cost of conformally coating the components.

Moreover, one skilled in the art will appreciate that the presentinvention is not limited to use in the preparation of electroniccomponents. Indeed, some embodiments of the present invention may beimplemented in any currently used conformal coating process utilized inthe preparation of any product (e.g., painting the metal surfaces ofautomobiles, appliances, road signs, etc.)

The conformal coating 130 is composed of a polymer into which metalnanoparticles are blended. The metal nanoparticles may be, for example,blended into the polymer using a high shear dispersion mixer or othersuitable technique known to those skilled in the art.

The gettering functionality of the metal nanoparticles binds and trapsthe target corrosive species (e.g., elemental sulfur in the air).Binding this corrosive species prevents the diffusion of the corrosivespecies to the underlying metallurgy. If just a polymer coating wasused, diffusion of the corrosive species would still occur. Polymercoatings only slow, but do not trap, the corrosive species. The metalnanoparticles, being a sulfur-getter, work by attacking thesulfur-sulfur bond in the corrosive species, breaking it and remainingcovalently bonded to it. Hence the corrosive species is trapped, whichprevents the further diffusion toward the surface of the electroniccomponent. This eliminates the possibility of the corrosive speciesreaching the underlying metallurgical surfaces of the electroniccomponent, and thus prevents corrosion of those metallurgical surfaces.

Preferably, the metal nanoparticles do not react with non-sulfurcomponents in the air (e.g., carbon dioxide) which would otherwisedeplete the availability of the metal nanoparticles for the targetreaction (i.e., reaction with sulfur components in the air). Thiscontrasts with enhanced RTV silicone rubber compositions known in theprior art that utilize an amino-bearing compound to prevent or retardelectronic parts encapsulated or sealed therewith from corrosion withsulfur-containing gas, in which the amino-bearing compound binds notonly with the sulfur components in the air but, disadvantageously, withcarbon dioxide in the air. Hence, the amino-bearing compound in suchprior art RTV silicone rubber compositions is quickly consumed by carbondioxide in the air and is not available to bind with sulfur componentsin the air. The amino-bearing compound in such prior art RTV siliconerubber compositions also disadvantageously binds to tin catalyst, whichis typically required in the formation of RTV silicone rubbercompositions.

Conventional gate resistors are typically coated with an overcoat of aconventional RTV silicone rubber composition that fails to prevent orretard sulfur components in the air from reaching the inner silverlayer. Hence, any sulfur components in the air will react with the innersilver layer to form silver sulfide, which is electricallynon-conductive. The silver sulfide formation (often referred to assilver sulfide “whiskers”) produces an electrical open at one or more ofthe terminations of the gate resistor and, thereby, failure of the gateresistor.

In contrast, the metal nanoparticle-containing polymer conformal coatingin accordance with some embodiments of the present invention is able toprotect the metal conductors from corrosion caused by sulfur components(e.g., elemental sulfur, hydrogen sulfide, and/or sulfur oxides) in theair. The metal nanoparticles in the conformal coating react with anycorrosion inducing sulfur component in the air and prevent the sulfurcomponents from reacting with the underlying metal conductors.

FIG. 4 is a flow chart diagram of a method 400 for producing a resistornetwork array that utilizes a metal nanoparticle-containing conformalcoating to protect metal conductors in accordance with some embodimentsof the present invention. Method 400 sets forth the preferred order ofsteps. It must be understood, however, that the various steps may occursimultaneously or at other times relative to one another. Method 400begins by providing a conformal coating composition including a polymerand metal nanoparticles blended with the polymer (step 402). Forexample, the conformal coating composition may be prepared by blendingthe metal nanoparticles into the polymer using a high shear dispersionmixer.

Method 400 continues by providing a resistor network array mounted on aprinted circuit board (step 404). For example, the resistor networkarray may include gate resistors electrically connected bysilver-bearing metal conductors (e.g., termination structures eachhaving one or more silver-containing layers). Thenanoparticle-containing conformal coating composition is applied ontothe printed circuit board to encapsulate the resistor network array(step 406). Preferably, the metal nanoparticle-containing conformalcoating composition is applied in an at least partially uncured state bydipping, spraying, spin-coating, casting, brushing, rolling, syringe, orany other suitable deposition process. Then, the metalnanoparticle-containing conformal coating composition is cured tothereby produce the metal nanoparticle-containing conformal coating(step 408). Generally, the process used to cure the metalnanoparticle-containing conformal coating composition will vary based onthe particular metal nanoparticle-containing conformal coatingcomposition used. For example, the metal nanoparticle-containingconformal coating composition may be cured in a conventional dryingprocess.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. For example, some embodimentsof the present invention are described above in the context ofprotecting metal conductors of electronic devices from corrosion causedby sulfur components in the air. One skilled in the art will appreciate,however, that the present invention can also apply to preventingcorrosion to any metal surface, such as the metal surfaces of anautomobile. Thus, while the present invention has been particularlyshown and described with reference to some embodiments thereof, it willbe understood by those skilled in the art that these and other changesin form and detail may be made therein without departing from the spiritand scope of the present invention.

What is claimed is:
 1. An apparatus, comprising: a substrate; anelectronic component mounted on the substrate; metal conductorselectrically connecting the electronic component; and a conformalcoating overlying the metal conductors and the substrate, wherein theconformal coating comprises a polymer and metal nanoparticles blendedwith the polymer, wherein the metal nanoparticles have an averagediameter within a range of 5 nm to 200 nm, and wherein the concentrationof the metal nanoparticles in the conformal coating is approximately 5wt % of the conformal coating and provides a level of electricalconduction sufficiently below the electric conducting percolationthreshold to avoid shorting of the electronic component and thesubstrate.
 2. An apparatus, comprising: a substrate; an electroniccomponent mounted on the substrate; metal conductors electricallyconnecting the electronic component; and a conformal coating overlyingthe metal conductors and the substrate, wherein the conformal coatingcomprises a polymer and copper nanoparticles blended with the polymer,wherein the copper nanoparticles have an average diameter ofapproximately 100 nm, and wherein the copper nanoparticles compriseapproximately 5 wt % of the conformal coating.